1. Field of the Invention
The present invention relates to inkjet recording apparatuses, and more particularly, it is related to an inkjet recording apparatus having a mechanism for correcting an uneven density when a recording operation of a continuous image is interrupted and is then resumed.
2. Description of the Related Art
An inkjet recording apparatus for recording an image by discharging ink towards a recording medium (recording material) such as a sheet of paper, a sheet of cloth, a plastic sheet, or an overhead projector (OHP) sheet on the basis of image data (recording data) is widely used as a recording apparatus having a function of a printer, a copying machine, or a facsimile machine, or as a recording apparatus (a printing apparatus) serving as an output device of a combined electronic device including a computer and a word processor, a workstation, or the like.
In order to meet a variety of requirements for recording media composed of new materials, in recent years, there has been developed a recording apparatus in which recording media composed of materials such as cloth, leather, unwoven cloth, and metal, other than normal recording media such as sheets of paper (including a thin sheet of paper and a sheet of converted paper) and thin plastic sheets (including an OHP sheet) are used.
Since the majority of the foregoing inkjet recording apparatuses are of a so-called serial scan type in which an image is recorded while a recording head performs scanning operations a plurality of times in a direction (main-scan direction) intersecting with the forwarding direction (the sub-scan direction) of a recording medium, and have a reduced size of a recording head unit and a low cost structure, a large number of inkjet recording apparatuses have been commercialized from various manufacturers.
In such an inkjet recording apparatus of a serial scan type, when a recording operation is performed, ink mist produced when ink is discharged from a recording head, splash mist produced due to an impact occurring when discharged ink reaches a recording material, or the like sometimes accretes on a discharge-port surface of the recording heads. Accordingly, there is a risk in that discharge ports of the recording heads are clogged with the accreted ink mist, thereby leading to a failure in discharging ink.
In order to solve the above problem, the foregoing recording apparatus is constructed such that the ink mist accreted around the discharge ports is wiped and removed by providing a wiping blade composed of a rubber-like elastic member such as polyurethane rubber and by moving the recording heads in a state in which the wiping blade abuts against the discharge-port surface of the recording heads. Such a discharge-recovery operation is called a wiping operation.
Also, since an image is formed by selectively discharging ink from the plurality of discharge ports of the recording heads during the recording operation, some of the discharge ports formed at the fronts of nozzles of the recording heads may remain in contact with the air without ink being discharged therefrom. In such nozzles, since ink in the nozzles gets evaporated and dried, and thus has an increased viscosity, the amount of discharged ink decreases and a discharge speed of ink decreases, thereby sometimes causing a discharge failure such as a wrong discharge direction. In order to remove the evaporated and dried ink having an increased viscosity, the discharge failure is prevented from occurring by discharging ink, unrelated to the recording operation, from the nozzles of the recording heads towards an object other than a recording medium. Such a discharge-recovery operation is called a preliminary discharge operation.
Also, when the recording operation of each recording head is continuously performed for a long time, the temperature of the recording head increases due to the heat stored when recording ink is discharged, which causes gas in a form of a bubble to be mixed in an ink holder (common ink chamber) or the like placed in the vicinity of the nozzle of the recording head. When the bubble is inflated to a certain extent, the bubble sometimes prevents ink from being fed to the nozzle, and resultantly from being normally discharged. In order to solve the above-mentioned problem, a cap composed of rubber or the like is disposed so as to abut against the discharge-port surface of the recording heads (i.e., the surface in which the discharge ports of the recording heads are formed), so that bubbles together with ink remaining in the vicinities of the nozzles are forcefully sucked and discharged via the cap. Such a discharge-recovery operation is called a sucking operation.
By performing such a discharge-recovery operation of the recording heads before or after an image-forming operation, or during a standby time midway through the image-forming operation and before the start of the subsequent scanning operation, ink is normally discharged from the recording heads, thereby preventing degradation of image quality and thus always forming a high-quality image.
In the meantime, when an image-forming operation such as a long-banner printing operation for forming an image in a large area is performed, sometimes it is required to perform such a discharge-recovery operation in order to always form a high-quality image even when the recording operation is interrupted midway through the image-forming operation.
More particularly, since thermal energy for continuously discharging recording-ink for a long time is supplied to the recording heads, the temperatures of the recording heads during the recording operation such as the long-banner printing operation are higher than those immediately after the start of the recording operation.
Accordingly, when the wiping operation serving as the foregoing discharge-recovery operation is performed during a standby time midway through the recording operation and before the start of the subsequent scanning operation, since no thermal energy is continuously supplied to ink during the wiping operation, and also the temperatures of the recording heads midway through the recording operation are higher than an ambient temperature around the inkjet recording apparatus, the temperatures of the recording heads immediately after the resumption of the recording operation are lower than those before the discharge-recovery operation due to heat radiation during the discharge-recovery operation.
Likewise, when the preliminary discharge operation serving as the foregoing discharge-recovery operation is performed during a standby time midway through the recording operation and before the start of the subsequent scanning operation, although thermal energy is supplied to ink during the preliminary discharge operation, since the amount of thermal energy continuously supplied to the recording heads is smaller than that during the image-forming operation, thermal radiation during the discharge-recovery operation has a large influence on the temperatures of the recording heads, and hence the temperatures immediately after the resumption of the recording operation are lower than those before the discharge-recovery operation.
Also, when the sucking operation serving as the foregoing discharge-recovery operation is performed during a standby time midway through the recording operation and before the start of the subsequent scanning operation, since no thermal energy is continuously supplied to the recording heads during the sucking operation, and also, in addition to heat radiation during the discharge-recovery operation, the sucking operation causes ink in an ink-feeding path for feeding ink to the recording heads to flow into the recording head, the recording heads are cooled down, whereby the temperatures of the recording heads immediately after the resumption of the recording operation are further lower than those before the discharge-recovery operation such as the wiping operation, or the preliminary discharge operation.
Unfortunately, such known discharge-recovery operations have the following problems which must be solved. A temperature of ink in the inkjet recording heads is very important to maintain the amount of the ink discharged from the recording heads constant. That is, a viscosity and a surface tension of the ink vary in accordance with the temperature thereof, thereby causing the amount of discharged ink to vary.
In order to solve the above problem, when the discharge-recovery operation of the recording heads is performed during a standby time midway through the image-forming operation and before the start of the subsequent scanning operation so as to normally discharge ink from the recording heads, to prevent degradation of image quality, and thus to always form a high-quality image, the ink temperatures of the recording heads immediately after the resumption of the image-forming operation become lower than those before the discharge-recovery operation, thereby decreasing the amount of discharged ink and thus decreasing an optical density of the image.
In addition, in the case where the recording operation is performed by transferring a large amount of image data to the inkjet recording apparatus with image-data transfer means such as an interface as in the case of the long-banner printing operation, when the recording operation is performed under a printing configuration in which the transfer rate of the image data is lower than the recording speed of the inkjet recording apparatus, sometimes it is required to temporally interrupt the recording operation until the image data is transferred to a host device and then to resume the recording operation after the finish of transferring the image data. On this occasion, since the length of a downtime of the recording operation depends on a processing speed of a CPU of the host device connected to the inkjet recording apparatus, a specification of the interface, and the amount of recording image data, a decrease in an optical density due to decreases in the temperatures of the recording heads caused by heat radiation during the downtime of the recording operation and a printing interruption of the recording heads is not uniquely determined but varies greatly depending on a user configuration.
Also, since the foregoing decrease in the optical density during the printing interruption takes place in mutually adjacent image-forming regions before and after the printing interruption, even a slight decrease in the optical density is significantly noticeable with respect to an actually printed image, thereby degrading image quality.
Meanwhile, in order to remove a strain caused by an ink drop accreted on the discharge-port surface of the recording heads, for example, Japanese Unexamined Patent Application Publication No. 6-328723 has disclosed a technique with which the discharge-port surface is cleaned by making ink in the nozzles to overflow towards the discharge-port surface, and then the temperatures of the recording heads are returned to those immediately before cleaning the discharge-port surface.
Although a recording apparatus according to the above-mentioned technique has a structure in which, after the discharge-port surface is cleaned, the temperatures of the recording heads are returned to those immediately before cleaning the discharge-port surface by recording-head heating means, unfortunately, due consideration has not been given to affects of the interruption of the recording operation and the length of the downtime of the recording operation, other than the above cleaning operation of the recording heads, on the optical density of an image.